Super low heating load, low temp, HE Water Heater?

Radman

Hey Scott, thanks for the response.
I think what everybody is missing here is the fact that when we use cont. circ on a slab, delta t's are really low. Maybe 5-7deg. So using a mod-con is great just so long as you add a buffer tank. Well my gripe is, now I've got a way overkill backup system, plus a buffer tank footprint that could be better served for solar storage.
I think I've got the solution though, Bradford-White's PDX unit has a 65k input for LP, de-rates to about 44k for our efficiency & altitude, and has side tappings. Plus it is direct vent and the terminations aren't too hideous looking. As far as the solar, the control is a Steibel Som 7si, and we are using a dual coil tank, piped separately for stratification. The Som 7 has a nice arrangement to alternate priority between the bottom & top of the tank. I've used a couple of these now and they are quite simple to set up. Since we have a flowmeter in the solar loop, we can perform heat balancing. The Som7 is very good at controlling stagnation, and has both a tank cooling & re-cooling option, as well as an option for cooling the header on a heat pipe array. I've pretty much got it covered. I think for smaller structures a small mod-con makes sense, but there are only 2 reasonable choices, and I don't like either product. The radiant will be controlled by a Tekmar 360, mated to a PAW k34 pump station & mix valve. It's such a simple system that putting a smart boiler in the fray just doesn't make sense to me at all. Especially since there are no recovery loads.

Radman_2

Super low load, low temp, Hot water heater???

Have a super low load house, 26kbtu @ -10degF, 7000ft elevation. Finished concrete floors, 98degF design day supply temp, (This house is greener than a tree-hugger in Birkenstocks at an Al Gore convention) Continuous circulation, and a Vitosol 300-20 collector with an 80g storage tank for DHW pre-heat. Originally I was spec'ing a full space heating compliment with 4 Vitosol 300-30 tube collectors ground mounted with a 62% solar fraction, but the budget bounced the space heat for now. Soooo, I need to come up with a heat source that is reasonably efficient, direct vent, and that can be vented concentric or have its intake & exhaust split to separate atmospheres.
Any ideas?

don_185

WTW

WTW..heatpump.

Brad White

Absolutely

Thinking the same thing. No local combustion, high COP's. Water temperature is right there, too. A good bet and no altitude issues regarding gas. Take a look at FHP although I am sure that there are others. Well drilling could be a challenge unless you have some deep meadow.

hr

Heat pump for a 26K load?

sounds a little pricey, but...

A 50K modcon would be one option.

A 40 gallon electric water heater (under 300 bucks) would work with two 5.5KW elements running.

Seems the more of them I work on the more it appears super tight ICF or SIPs heat themselves with lights, appliances, and occupant BTU's.

hot rod

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Mike T., Swampeast MO

Free-standing catalytic, soapstone wood stove?

Radman_2

No water for 700ft

Yeah, heat pumps are out. The money would by more solar than you could use. I was going to go a Propane WH, direct vent with an external plate HX. The water heater would be very stable. I thought tankless too, but it seems overkill. So is the mod-con

Radman_2

Yeah, theres one of those here too, but only upstairs. 26k, and its about 80%eff. At 7000ft, its only putting out 16,000btu though.

Andrew Hagen

Heat Sources

If gas, I would use one of the Bradford White Combi2 water heaters for dual use. They aren't as efficient as a mod/con, but the load is so small.

If electric, I would use a Laing EPR Heater.

hr

Yes, the Laing would be an excellent

choice also. I installed yet another on a small bathroom job this week.

hot rod

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Mike T., Swampeast MO

Can you define "upstairs"? 2nd above-ground floor or only above-ground floor over a basement? Or perhaps something else?

Mike T., Swampeast MO

Sounds like the boiler will wind up being a virtual emergency backup.

Considering the start-up and life-cycle expenses of geo or solar, the "greenest" source could well be a plain old electric water heater just as Hot Rod suggested.

Perhaps a tiny "instant" gas-fired water heater if you use a heat exchanger to turn a low-flow/high temp gain source into a high-flow/low temp gain provider...

Radman_2

Upstairs defined

Upper level is 900sf , lower is the same 50% above & below grade blasted into a mountainside of solid rock.

Radman_2

Solar is the greenest by far...

Well, Electric isn't as productive per kw as propane, and since there's no nat. gas, I'd have to stay with a tank water heater. The Combi Core suggested before is expensive, and not a direct vent sealed combustion unit, to that's out.

Radman_2

50k what Hot Rod?

Were you suggesting the Contender? I could be strung up alive for putting one of those in here in my 'hood.
Sorry bro, it's all sourbraten & spaetzle for me.

Andrew Hagen

sealed combustion

There is a power vented version, but it is not sealed combustion.

Maybe a small mod/con is the best choice to achieve sealed combustion operation. To avoid short-cycling, you could use a dual coil indirect and just use the 6/24 to heat the DHW tank.

Radman_2

Laing EPR

That was our choice with the large solar support, but for this load I don't want electric. Sweet unit though. I also use their DC pumps for single panel stand alones.

Radman_2

The other coil for the heating zones?

I'd love to go modcon, but there is nothing I would use that is small enough. The other coil would be a poor HX for the heating side though don't you think?

Andrew Hagen

Sure, why not?

It seems like it would be decent to me. It will take the input of the boiler and then some, so it should certainly work fine for the heating zones. Obviously temperature differential will be everything. Plus with the DHW pre-heat you would get a *little* solar assist with the heating. Ok, that might be a bit of a stretch considering the size of the collector and that piping strategy.

Radman_2

Pretty low delta

Well, our design day delta t would be about 22deg, so thats not wide enough to get a great pull off of any dual coil I would use, like a Steibel. To get the delta t up I'd have to make the boiler keep the tank at around 150-160. You'd get good hx but the boiler would not condense, so now I've got a tankless water heater that costs 3 times as much. I could put the boiler on ODR along with the floor mixing...that seems a little excessive though, and would still not condense in colder weather. I thought about a Turbomax 26, but Yowch!

Andrew Hagen

hmm

Just throwing out ideas. The more I thought about it, that probably isn't the best idea. What we really need is a small mod/con. A Vitodens 3/12 would be nice.

The Crown CWD is available in a 60MBH version.

hr

The HTP Phoenix

tank style mod con. DHW and a plate HX for DHW boost.

hot rod

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scott markle_2

why rule out the mod con?

Yes, a smaller vitodens should be brought to the N. American market. To my knowledge the pinnacle T-50 or contender MC-50 are the smallest mod cons available. "boy the contender is ugly though"

In my opinion it would be a mistake to use electrical resistance to heat this house. Obviously a large effort has been made here to create something uniquely efficient. One problem with efficiency in general is that gains are often lost to user behavior changes. If we don't feel a little pinched by costs it's unlikely we will put on that thick sweater or step out of that luxuriously hot shower a bit sooner. These are cultural issues created by decades of entitlement. By the way Al Gore's house is a total energy hog http://www.tennesseepolicy.org/main/article.php?article_id=367.

Using electric resistance would be affordable in this house but would absorb a good deal of it's green assets. My own 1500sq. ft house has a similar heat loss. I heat with a vitodens 6-24. I have complained about short cycling within this forum, because I believe that control based solutions exist for this problem. The vito is equipped with a potentiometer on the control board that will limit the output of the boiler in heating mode. As an experiment I have adjusted this to it's lowest setting aprox. 22k out. During the coldest parts of this heating season temperatures hovered very close to design (0) for days in a row. The boiler ran almost continuously at supply temps. slightly over 100 f. This gave me a pretty clear sense of my actual heat requirements (not including internal gains). Basically In heating mode my boiler is essentially an on or off machine. While I'm still seeking a solution to the short burns, this system is working very well.

A potential drawback to matching a boilers output to the load of a highly efficient building envelope is that it will require an oversized dhw storage tank. The larger the tank the more standby loss. I don't see why a T-50 would be a problem for this structure. Equip it with a boiler control that includes a load dependent differential and I don't think short cycle will be a problem.

Question on the solar, will you be using a preheat tank and indirect as opposed to a two coil stratified temp. tank? If so are there concerns about temperatures in preheat tank not being hot enough to prevent potential pathogens. I understand that smart systems use photo sensors and schedules to allow tank temps. to drop in anticipation of solar gain. What sort of controls are you planing for the solar. Viessmann incorporates connections on the vitodens for interface to a solar control system. How many boiler company's also manufacture solar and geoexchange systems and conduct research on fuel cell heating ?

You might consider a gfx heat recovery on the waste water. This is a real simple way to reduce dhw production costs.

Keep it green, as annoying as the hypocrite birkenstock tree hugger crowd can be sometimes, these early adopters are helping advance important planet saving technology, and the ones with money could be great patrons of the hydronic arts.

Andrew Hagen

mod/con

My personal opinions do color my judgement when selecting a small mod/con. I have always felt the Munchkin and now the Contender are designed much more around cost than quality. I could not imagine taking a house with Vitosol 300 collectors and installing a Contender.

I agree with Scott, if the budget is there, go with the 6/24. I would add that a small buffer tank in the return pipe may be helpful since this will certainly not need a low loss header. Obviously it is not perfect, but from what I can tell, small mod/cons are not yet on the horizon.

Unknown

there are two things to consider; sure, your max load might only be 26kBTUs/hr at design temp, but how many heating degree days are you servicing?

Sounds like it's probably a lot.

Munchkin T50, or if electricity makes sense I would consider an electric boiler or WH as HR suggests. Just because max load is low doesn't mean that efficiency doesn't count.

Radman

Degree days

Hey Rob,
Don't know about the degree days, our charts are woefully inaccurate. I really don't like the whole degree day system, I think it is antiquated. This is why I design the way I do in my area, since weather variable is so great. We look at NOAA & NASA weather data with variable updates to determine the actual KW usage. These green built structures are very good at stacking heat. Hot Rod was on to it when he commented about the way they heat just by lighting, occupant, & computer BTU's. If you go by degree day numbers published by NOAA, you will oversize the heating for a structure like this.